1. Field of the Invention
The present invention relates generally to radio geolocation and particularly to self-calibrating receivers for position location using periodic codes in such broadcast digital transmissions as broadcast digital television (DTV) signals.
2. Description of the Prior Art
Among different radio geolocation and navigation systems, there are two important systems in wide use today. One is the 100 kHz Long Range Navigation-C (LORAN-C) system which evolved to the present form in the mid-1950s. It uses terrestrial radio transmitters to provide navigation, location, and timing services for suitably equipped air, land and marine users, civil and military alike. A LORAN-C receiver measures the difference in times of arrival of pulses transmitted by a chain of three to six synchronized transmitter stations separated by hundreds of kilometers. There are many LORAN chains of stations around the globe. Modernization effort is underway to enhance the accuracy, integrity, availability, and continuity of the LORAN system, known as Enhanced LORAN or eLORAN for short.
The other is the increasingly popular satellite-based Global Positioning System (GPS). Fully operational since 1994, the GPS relies upon a constellation of twenty-four satellites in six different orbital planes around the earth for position location, navigation, survey, and time transfer. Each satellite carries a set of ultra precise atomic clocks and transmits pseudo-noise code modulated signals at several frequencies. By tracking four or more satellites, a user can solve for the variables of longitude, latitude, altitude and time to precisely determine the user's location and calibrate its clock. More details are provided in the books entitled, Global Positioning System: Theory and Applications (Vols. I and II), edited by B. W. Parkinson and J. J. Spilker Jr., AIAA, 1996; Understanding GPS: Principles and Applications, edited by E. D. Kaplan, Artech House Publishers, 1996; Fundamentals of Global Positioning System Receivers—A Software Approach, by J. B. Y. Tsui, John Wiley & Sons, Inc., 2000; and Global Positioning System, Signals, Measurements, and Performance, by P. Misra and P. Enge, Ganga-Jamuna Press, 2001.
Despite of its increased popularity, GPS cannot function well when the line-of-sight view between a receiver and a GPS satellite is obstructed due to foliage, mountains, buildings, or other structures. To satisfy the requirements of location-based mobile e-commerce and emergency call location (E911), there have been ongoing efforts so as to improve GPS receiver sensitivity to operate on GPS signals of very low power level. One approach is the assisted GPS (AGPS). The AGPS approach relies upon a wireless data link to distribute, in real time, such information as time, frequency, navigation data bits, satellite ephemeredes, and approximate position as well as differential corrections to special GPS receivers equipped with a network modem so as to reduce the uncertainty search space, to help lock onto signals, and to assist navigation solution. This approach, however, comes with a heavy price associated with installing and maintaining the wireless aiding infrastructure and services.
Another approach strives for weak GPS signal acquisition without network assistance. This is done by enhancing the sensitivity of standalone GPS receivers with extended coherent integration. As an example, the block-accumulating coherent integrating correlation at extended length (BACIX) algorithm is disclosed in the patent application Ser. No. 11/173,894, filed Jul. 1, 2005, entitled, Method and Device for Acquiring Global Navigation Satellite System (GNSS) Signals of Very Low Power Level by the present inventor.
Amid the ongoing replacement of analog National Television System Committee (NTSC) television signals by an Advanced Television Systems Committee (ATSC) digital television (DTV) signal, there has been a considerable amount of efforts devoted to the use of DTV signals for position location, thus serving as a complement to and/or a substitute for GPS. This is exemplified by the U.S. Pat. No. 6,861,984, entitled, Position Location Using Broadcast Digital Television Signals, by M. Rabinowitz and J. J. Spilker Jr., issued Mar. 1, 2005.
Designed primarily for indoor reception, DTV signals exhibit several advantages. It is much higher in power (40 dB over GPS) and at lower and more diverse frequencies (nearly half of the spectrum between 30 MHz and 1 GHz). The geometry offered by a network of terrestrial DTV transmitters is superior to what a satellite constellation can provide. As such, it has better propagation characteristics with greater diffraction, larger horizon, and stronger penetration through buildings and automobiles. DTV signals have a bandwidth of 6 to 8 MHz, which is much wider than the primary lobe of GPS C/A-code (2 MHz), thereby minimizing the effects of multipath and permitting higher accuracy tracking.
With DTV transmitters fixed on the ground, their lines of sight to a user changes very slowly, only adding a small amount of Doppler shift to a DTV signal frequency. This allows the signal to be integrated over a long period of time, thus easing the task of acquisition and tracking of a weak signal considerably. As a further benefit, the component of a DTV signal that can be used for timing is of low duty factor (e.g., 1 of 313) in contract to GPS wherein the ranging code is repeatedly transmitted and has to be tracked continuously.
However, one inherent technical difficulty faced by position location using broadcast digital transmissions (BDT) such as DTV signals is the clock bias and drift of the signal timing source at a transmitter, which are unknown to a user initially. Although it may be possible to have all DTV stations use ultra-precise atomic clocks or GPS disciplined clocks, the synchronization of all signal transmissions across a large region is a daily operational challenge. It may also be possible to time-tag all transmissions and embed the clock offset information in the broadcast signals for all stations in a given region. However, these approaches require coordinated involvement of local DTV operators who are not in time transfer but rather broadcasting business.
Many inventions exemplified by the U.S. Pat. No. 6,861,984 by M. Rabinowitz and J. J. Spilker Jr. mentioned earlier make use of base stations, location servers, and monitor units to calibrate the DTV transmitters' timing biases and to provide the calibration data to mobile users via dedicated data links. The double difference technique may also be employed in these systems for positioning. There is a significant cost associated with installing and maintaining the infrastructure of base stations, location servers, and monitor units on a large scale. A user has to subscribe to a service coverage in addition to special equipment for the service signals.
Clearly, a user risks to experiencing service discontinuity when moving from one region (or a country for the matter) to another without a global service network in place or a valid global subscription. These prior-art approaches further prevent broadcast digital transmission (BDT) signals from being used for military applications as signals of opportunity (SOOP). A need therefore exists for a self calibrating position location system using broadcast digital transmissions such as DTV signals that does no require the service from external base stations, location servers, and monitor units. This need is met by the present invention as described and claimed below.